The present invention generally relates to a variable wavelength monochromator and, in particular, relates to such a monochromator including a transversely movable slit to vary the wavelength, which slit is positioned in the path of rays of two dispersive elements.
In a conventional monochromator, a light beam to be spectrally dispersed is directed upon an entrance slit. The light beam is directed from that entrance slit to a dispersive element, for example a grating or a prism. The dispersive element spreads, or fans, the incident radiation as a function of the wavelength. The radiation is thus directed in different directions as a function of the wavelength. An image of the entrance slit is generated by the imaging optical means by means of the spectrally fanned light beam. This image of the entrance slit is correspondingly spectrally dispersed and forms a spectrum. A stationary exit slit is arranged in the plane of this spectrum through which exit slit radiation of a certain wavelength emerges from the monochromator.
Ordinarily, to vary the wavelength of the emerging radiation the dispersive optical element is rotated. This optical element may be, for example, a Littrow-mirror in a prism monochromator or the grating in a grating monochromator. To obtain a linear wavelength, or wavenumber, graduation the rotation has to be carried out in accordance with a determined function of the desired wavelength. In a grating spectrometer, for example, the rotation is carried out in accordance with a sine function. Such non-linear drives are complex and expensive.
If, in addition, a spectral range is to be scanned several times it becomes necessary to return the rotatable optical element to its initial position. The time between scans, i.e., the frequency of such a periodic scanning of the wavelength range, is greatly limited thereby.
In addition to the desired radiation, radiation of undesired wavelengths (stray light) also emerges through the exit slit. One conventional device employed to reduce this stray light component is a double monochromator. Such double monochromators usually comprise a first dispersive element onto which the light is directed from the entrance slit. The first dispersive element and related imaging optical means generate a spectrum in the plane of an intermediate slit. This intermediate slit only transmits light of a certain selected wavelength which is, however, superimposed by a certain stray light component. The intermediate slit represents the entrance slit for a second monochromator having a second dispersive element. The light, which is again fanned by the second dispersive element, is collected as a spectrum in the plane of an exit slit. The exit slit transmits from this spectrum light of the same wavelength as that transmitted by the intermediate slit. Again, stray light may develop by scattering and deficiencies in the components. However, as substantially only light of the desired wavelength initially passes through the intermediate slit out of the pre-monochromator into the main monochromator, the final content of stray light represents only a small proportion of an already small incident proportion. The wavelengths of a double monochromator are set by displacing optical components, in this case components of both pre- and main monochromators have to be displaced synchronously with the desired wavelength.